The interaction of acetaldehyde and formaldehyde in a sodium hydroxide aqueous solution results in formation of an equimolar sodium formate as well as pentaerythritol. This sodium formate must be separated from the pentaerythritol in an efficient manner. In general, the reaction solution is condensed and cooled for separation into solid pentaerythritol containing only a small amount of sodium formate and a condensed solution of liquid pentaerythritol which contains a relatively large amount of sodium formate. The solid pentaerythritol can be converted by a simple operation into pentaerythritol crystals with high purity. However, it is difficult to separate pure pentaerythritol crystals from the condensed solution containing a large content of sodium formate and it is uneconomical to discard the condensed solution as it is.